Coils in mechanical relays generate heat. When a relay is activated, the relay needs large current to pull in the armature. Once the armature is pulled in, only a small current is needed to hold the armature in place.
Relay manufacturers design relays such that they can operate under various operating scenarios. It is known that coil resistance increases with temperature. Instead of taking into account the actual temperature, current supplied to operate the armature of the relay is operated at above normal requirements to ensure operation at all temperatures. In some cases, during normal operating conditions current supplied to operate the armature can be more than double the requirement (i.e., to accommodate for high ambient air temperatures). The excess energy is then dissipated as heat. This excess heat generated by the relay coil can cause thermal problems for other electrical components. For example, power distribution center modules (PDCs) for a vehicle can include more than twenty relays. The twenty relays can provide enough heat to affect the operation of other electrical components within the vehicle.